Dopamine is an important neurotransmitter in the central nervous system (CNS), and also has several important roles in the peripheral nervous system such as in the control of supply of blood to the kidneys and in autonomic ganglion transmission.
It is now widely accepted that dopamine receptors in the CNS can be divided into two general categories, designated D-1 and D-2 receptors. The division was originally based on biochemical and pharmacological differences between the two receptor types. Recently, further evidence which supports this division has come from study of the molecular biology of dopamine receptors in the CNS. The dopamine D-1 receptor is linked to the enzyme adenylate cyclase through a stimulatory G protein such that stimulation of this receptor by dopamine or a dopamine D-1 receptor agonist causes an increase in the production of 3',5'-cyclic adenosine monophosphate (cAMP). The D-2 receptor, on the other hand, also regulates important functional activity within the CNS, although the biochemical events which follow stimulation of this receptor by dopamine or a D-2 receptor agonist are not as well understood. Autoreceptors on dopaminergic neurons which have the pharmacological properties of D-2 receptors appear to control the firing rate of these cells as well as the release of dopamine from the nerve terminals. It is also known that stimulation of the D-2 receptors in the intermediate lobe of the pituitary gland causes a decrease in cAMP production and that stimulation of the D-2 receptors on the mammotrophs of the anterior pituitary gland suppresses prolactin secretion. Dopaminergic neurons are also affected by and interact with other neurotransmitter systems in the CNS. For example, D-2 receptors on the cholinergic interneurons in the striatum (one of the components of the basal ganglia) regulate the release of acetylcholine from these cells.
Dopamine involvement has been proposed for several diverse neurological disorders such as Parkinson's disease and schizophrenia. The putative roles of the two types of dopamine receptors differ in these disorders.
One neuropathology involving dopamine is Parkinson's Disease. Dopamine occurs at high concentration within the nerve terminals in the basal ganglia of the mammalian brain. In the early 1960's, the loss of striatal dopamine was established as a chemical marker of Parkinson's Disease. This deficiency is still thought to be primary to the etiology of the disease state.
L-DOPA (3,4-dihydroxyphenylalanine), which is used in conjunction with a peripheral aromatic amino acid decarboxylase inhibitor and often supplemented with anticholinergic agents, has been shown to be useful in the treatment of Parkinson's Disease. It is theorized that the response to L-DOPA is a result of the conversion of L-DOPA to dopamine within the striatum, and that the response is linked to stimulation of both the D-1 and D-2 receptors.
The success of L-DOPA therapy has led to the testing of other compounds capable of mimicking the post-synaptic receptor actions of dopamine. Such direct-acting agents might offer the therapeutic advantages of greater potency, increased duration of action, or fewer side effects over L-DOPA. For example, bromocryptine, the direct-acting dopamine agonist most widely used in the treatment of Parkinson's disease, lowers the amount of L-DOPA required to achieve the maximal therapeutic response and allows for a delay in the onset of L-DOPA therapy. However, the response to bromocryptine alone is not as great as that observed with L-DOPA.
Dopamine has been used in the treatment of shock, congestive heart failure and renal failure. Stimulation of the peripheral DA-1 receptors causes vasodilation, particularly in the renal and mesenteric vascular beds where large numbers of these receptors are found. The utility of dopamine has been limited, however, by its ability to cause vasoconstriction at higher concentrations, presumably due to its secondary effects on adrenergic receptors and by its emetic effects due to peripheral DA-2 stimulation. Agents selective for the peripheral DA-1 receptors may offer significant advantages over currently used treatments for these and other disorders.
A second neurological disorder in which dopamine has been implicated is the psychosis schizophrenia. The psychoses are serious psychiatric illnesses characterized by abnormal behavior which may include delusions, hallucinations, violence, mania and serious long-lasting depression. Schizophrenia is the most common psychosis and involves disturbance of thought processes, hallucinations and loss of touch with reality. The theory of schizophrenia as a disease of the CNS was first formalized by Kraepelin and Bleuler in the early 1900's. It was not until chlorpromazine was discovered by Delay and Daniker in the early 1950's, however, that effective drug management of this disease was possible.
The pioneering work of Carlsson and others led to the now widely-held dopamine theory of schizophrenia. According to this theory, schizophrenia is caused by an excess of dopamine in the brain. Several line of evidence support this hypothesis. For example, chronic abuse of stimulants such as amphetamines, known to enhance dopaminergic activity in the brain, can lead to a paranoid psychosis that is almost indistinguishable from classic paranoid schizophrenia. The mechanism-of-action proposed for drugs with antischizophrenic activity is the blockade by these compounds of the dopamine receptors, and consequently, the prevention of excess receptor stimulation. In the mid 1970's it was observed that virtually all of the currently used antipsychotic agents could displace radiolabeled haloperidol (a dopamine antagonist) from striatal dopamine receptors with a good correlation between average effective clinical dose and drug binding affinity.
Unfortunately, the currently available antipsychotic agents frequently produce undesirable side-effects, the most common of which are the so-called extrapyramidal effects that include bizarre involuntary movements and Parkinson-like effects. Sedation and hypotension are also common side effects. Because of these often severe side-effects and the high incidence of patients unresponsive to currently available drugs, more potent and selective agents are needed.
Published evidence suggests that dopamine also has a central role in the brain's reward system. In particular, it has been reported that animals trained to self-administer cocaine will increase their consumption of this drug after treatment with either a D-1 or a D-2 receptor antagonist. It was proposed that the animals would increase the amount of cocaine administered in order to maintain the elevated dopamine levels responsible for the drugs euphorigenic and reinforcing properties. Because of this interrelationship, dopamine antagonists are potentially useful for the treatment of drug abuse and other addictive behavior disorders.